I would wait for GM to rework dual boot.
The old dual boot in Dorimax was designed to use Preload to hold the 2ndROM system partition. Since this is now being exploited to hold part of the bigger JB ROMs which don't fit into the original 512mb system partition GM will use a method similar to the S3 where the system partition for 2ndROM is stored in an image file. I'm guessing the overflow of system in preload will be stored in a preload image file as well.
Remember dual boot works because the kernel mounts these image files as if they were the original partitions. Preload used to hold the 2ndROM system partition and since both preload and the system partition are 512MB preload was mount as system for 2ndROM booting.
With Jelly Bean dual boot will need to make image files to be mounted as both system and preload in order to mimic the new split-partition regime.
Sent from my GT-I9300 using Tapatalk 2